Saturday, December 20, 2008

Cerberus, a RepStrap machine: a start at documenting my thoughts and efforts

Greetings all,

This is the beginning of my 'blogged "build journal" of my efforts to build a RepStrap machine.(Please bear with me; this is my first blog; I'm learning as I go.)

What is a RepStrap? A RepStrap is a machine to bootstrap into RepRap capability. A RepRap is a Replicating Rapid Prototyping machine -- a "three-dimensional-printer," particularly one that can print many of its own parts, and whose design is open-sourced. The RepRap project was started by Adrian Bowyer, in the Mechanical Engineering Dept. of Bath University, UK.For background details, see the reprap website.

Why build a RepStrap and/or RepRap machine?

For the fun,

For the technical challenge (motivation to learn new things),

To justify buying a bunch of tools I've always wanted, anyway!

What is/will be different about Cerberus vs. other already-extant RepStrap machines?

Cerberus is *my* RepStrap, built the way I want it (or as close as my skills, time, tools and budget permit.)

Cerberus will be capabile of both additive fabrication and "conventional" subtractive machining. Note: several other repStraps do this, notably nophead's HydraRaptor and Forrest Higgs' Tommelise (among others), so this notion isn't new here.

Cerberus' design is not constrained to be buildable with simple tools as is the Darwin RepRap, or the McWire-based mechanism often used for RepStrapping. (I'm explicitly going for performance/versatility rather than ease of development/adoption.)

Why name it Cerberus?

Cerberus is the name of the three-headed dog that guarded entry into to land of the dead in Greco-Roman mythology. My RepStrap will (eventually) have multiple tool "heads" one for additive fabrication, another for subtractive fabrication (milling), and a third for measuring/probing (~CMM) So that's three, and nophead already used Hydra in the name of his machine. Iff I get to the point of adding a forth head, I may have to rename it. (I considered using the name Janus, a two-headed Greco-Roman deity, but upon investigating, I decided that I liked Cerberus better. Proteus (also from mythology) was also a candidate, for awhile. (Proteus was also the name of the submarine that was miniaturized in the movie _Fantastic Voyage_, one of my childhood favorites. But since this machine won't really change form or size, the name isn't that good a fit. )

Brief outline of Cerberus' design:

The Cartesian(X, Y, Z) mechanism

I've based the horizontal (X, Y) degrees of freedom on a "compound table" (AKA XY table, or mill/drill table) from Shars Similar devices are available from other vendors (e.g. Grizzly, Enco, or Palmgren also smaller ones.) Mine is an import from PRC, and the typical caveats re quality apply, though (after dissasembly, cleanup, re-assembly, and adjustments), I think it'll do what I need, and it's good bang/$.

After dis-assembly/cleaning, but before re-assembling it, I lapped the linear bearing surfaces (slideways, AKA ways) of the table. There is (to put it mildly) considerable controversy on whether this is a good idea, or a terrible one. "Purists" argue that the proper way to improve ways is by scraping them in (and using a reference surface and prussian blue to find the high spots to scrape.) I agree that (all things being equal, this is the most accurate method.) However, my goal was not mainly to improve geometric accuracy, but to reduce the friction, and thus reduce the torque my actuators (stepper motors) would require to move the table. I compromise was to lap the ways as carefully as I could, using a machinst's (6") parallel to hold the carbide paper flat (at least for local, 6" regions being lapped.) I used a light oil with the carbide and inspected carefully for any carbide grains embedded in the cast iron. (Some advocate using less agressive, non-embedding abrasives; I wasn't patient enough to wait for a mail order; I had the #400 carbide in hand.)

I made mounting brackets (out of some scrap Aluminum) to support the stepper motors and couple them to the Acme screws. These may need some iteration; the stepper motors can drive both X and Y, but only to ~50% of the motors' no-load speed. The residual backlash is also not perfect, but may do for now. If I re-design these, I will incorporate better thrust bearings, instead of the solid bushings I made with scrap brass and stainless I had handy.

For the vertcal DOF, I was planning to use a smaller linear stage (5" travel) I have in my junk box. Now that I have the XY table, I'm starting to think I should get something a bit sturdier, and maybe with a bit more travel. Stay tuned.

Additive toolhead (thermoplastic extruder)

TBD

Subtractive Toolhead

I bought a rotozip (model RZ20) to use as my tool spindle. This has some features I wanted:

Variable speed (though only ~factor of two, 15 -- 30,000 rpm.)

A three-jaw chuck instead of collets, this permits use of tools with a range of shank diameters, e.g. drill bits.

A removable handle -- I hope to either clone the latches, or cannibalize the handle to make a mount for it.